1. Field of the Invention
The present invention provides a method for employing tagged magnetic resonance imaging and associated angle images for determining two-dimensional or three-dimensional strain, small displacements, creating synthetic tag lines and optical flow processing.
2. Description of the Prior Art
The advantageous use of magnetic resonance imaging wherein a patient or object is placed within a magnetic field with alternating generation of RF pulses and gradient pulses serving to excite nuclei within the area of interest and cause responsive emission of magnetic energy which is picked up by a receiver and may be processed by computer means followed by recording, display or production of hard copy of the results has long been known. See, generally, Atalar-McVeigh U.S. Pat. No. 5,512,825 and Conturo-Robinson U.S. Pat. No. 5,281,914, both of which are assigned to the owner of the present invention, the disclosures of which are expressly incorporated herein by reference.
It has been known to employ two sets of tagging planes oriented orthogonal to the image plane in imaging two-dimensional heart wall motion with magnetic resonance imaging through spatial modulation of magnetization (SPAMM) approaches. See U.S. Pat. Nos. 5,054,489, 5,111,820 and 5,217,016. See also, Axel et al., MR Imaging of Motion with Spatial Modulation of Magnetization, Radiology, 171:841-845, 1989 and Axel et al, Heart Wall Motion: Improved Method of Spatial Modulation of Magnetization for MR Imaging, Radiology, 172(1):349-350, 1989.
It has been known in connection with magnetic resonance tagging to employ image processing techniques to detect tag features and subsequently combine the features into a detailed motion map related to displacement and strain with subsequent interpolation being employed. See, for example, Young et al., Three-Dimensional Motion and Deformation with Spatial Modulation of Magnetization, Radiology, 185:241-247, 1992 and McVeigh et al, Noninvasive Measurements of Transmural Gradients in Myocardial Strain with MR Imaging, Radiology, 180(3):677-683, 1991. These approaches are not automated as they require some manual intervention.
It has also been known to employ optical flow methods in respect of magnetic resonance tagging image sequences. See, generally, Prince et al., Motion Estimation from Tagged MR Image Sequences, IEEE Trans. on Medical Imaging, 11(2):238-249, June 1992, Gupta et al., On Variable Brightness Optical Flow for Tagged MRI, Technical Report, 95-13, JHU/ECE, 1995, and Gupta et al., Bandpass Optical Flow for Tagged MR Imaging, Proceedings of the IEEE International Conf. on Image Proc., Santa Barbara, 1997. In such approaches sinusoidal tag patterns are employed instead of saturated planes. Image brightness gradients are features which together with temporal derivatives estimated from image pairs can be used to provide dense motion estimates generally referred to as xe2x80x9coptical flow.xe2x80x9d Such approaches require regularization to compensate for the fact that the brightness gradients contain information about motion solely in the direction of the gradient.
U.S. Pat. No. 5,275,163 discloses the use of magnetic resonance imaging in monitoring motion of a part of an object. Pulse and gradient sequences are applied in pairs with spatially differing tagging patterns and subtraction being employed to form a tagged image.
U.S. Pat. No. 5,352,979 discloses observing a phase angle response of volume elements in a slice or volume and imaging occurring before and during perturbations caused by external stimuli.
U.S. Pat. No. 5,379,766 discloses quantitative motion evaluation of a portion of an object by employing a high contrast tagging grid for detection of tagging patterns. U.S. Pat. Nos. 5,315,248 and 5,545,993 disclose tracking of motion.
It has been known to employ planar tag analysis in magnetic resonance imaging. It has also been known to employ such approaches in connection with the analysis of myocardial motion. Such prior art methods typically involve extraction of motion from these images through displacement vectors or strain patterns and involve tag identification and position estimation followed by interpolation.
Phase contrast magnetic resonance imaging has also been known. It provides a method for directly measuring motion by measuring a property sensitive to velocity and reconstructing velocity fields with strain being computed by employing finite differences. One of the problems with these two approaches is that planar tagging images cannot be accurately analyzed automatically. Phase contrast images, while capable of being analyzed automatically, tend to have a low signal-to-noise ratio leading to unacceptable results.
In spite of the foregoing known systems, there is lacking an effective means for employing magnetic resonance imaging systems to compute strain directly in either as planar strain or three-dimensional strain tensors automatically and with great accuracy. There is also lacking the ability to employ optical flow calculations without regularization as employed in conventional optical flow.
The term xe2x80x9cangle imagexe2x80x9d as employed herein refers to the phase of an image corresponding to an isolated spectral peak in a SPAMM-tagged magnetic resonance image.
The above-described need has been met by the present invention. A method of measuring motion of an object by magnetic resonance imaging including applying a pulse sequence to spatially modulate a region of interest of said object. At least one spectral peak is acquired from the Fourier domain of the spatially modulated object. The inverse Fourier transform information of the acquired spectral peaks is computed. The angle images are computed from the spectral peaks. The angle images are employed to measure motion of the object.
The method may employ a SPAMM pulse sequence as the pulse sequence.
The invention may also be employed to determine small displacements, create synthetic tag lines and facilitate optical flow processing.
The invention is particularly suited to use on a heart and may be employed to evaluate myocardial ischemia and myocardial infarctions in terms of both detection and quantification.
It is an object of the present invention to provide an improved method for rapid and accurate visualization of motion of an object using tagged magnetic resonance images of an object.
It is another object of the present invention to provide such a method which employs isolated spectral peaks in SPAMM-tagged magnetic resonance images.
It is an object of the present invention to provide an improved means for employing two-dimensional or three-dimensional 1-1 SPAMM tag patterns in magnetic resonance imaging measurement of motion of an object.
It is a further object of the present invention to provide such a system wherein angle images are acquired from two or more spectral peaks of the Fourier transform information and are employed to provide planar strain or tensor strain computations.
It is a further object of the present invention wherein angle images may be employed to make such computations automatically and rapidly.
It is another object of the present invention to use the method to detect small displacements, create synthetic tag lines and to facilitate optical flow processing without regularization.
It is a further object of the present invention to generate such angle images automatically.
It is yet another object of the present invention to produce angle images directly from the Fourier data without requiring production of conventional magnetic resonance images.
These and other objects of the invention will be more fully understood from the following description of the invention on reference to the illustrations appended hereto.